JP2004-036538A, published by the Japan Patent Office in 2004, discloses an internal combustion engine that causes an air-fuel mixture to undergo compression ignition combustion (self-ignition combustion).
The internal combustion engine according to the prior art forms a homogeneous air-fuel mixture of gasoline in a combustion chamber. The internal combustion engine then injects hydrogen gas, which has a comparatively high octane value and does not self-ignite easily, in the vicinity of top dead center of a compression stroke to form a hydrogen-air mixture on the periphery of a spark plug. The hydrogen-air mixture is subjected to spark ignition combustion by the spark plug, and homogeneous air-fuel mixture in an unburned region is compressed as a result of combustion expansion of the hydrogen-air mixture so as to undergo self-ignition combustion.